Different feedstock results in different nutrient profiles for Black Soldier Fly Frass

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Black Soldier Fly (BSF) larvae produce a by-product known as frass, which is essentially their excrement. This frass can then be used as a granulated fertiliser product. This fertilizer is not only effective, low-cost, and slow release but also safe to handle, store, transport and apply. It can even be used in organic agriculture as the chemical composition is almost zero. The composition of this frass, however, varies dependent upon the types of waste consumed.

BSF derived fertiliser is primarily rich in Nitrogen because of the content of the animal and plant waste it generally decomposes. BSF harvested at larval stage had five major minerals comprising of Phosphorous, Magnesium, Sodium, Potassium, And Calcium.

Some general conclusions on the variance of Frass/BSF larvae composition have been drawn through our research:

  • The main differences appear to be between substrates (i.e eggs, pork or mix) and the product used (i.e frass or larvae).
  • Despite some statistical differences there is no meaningful influence of the feed rate on nutrients. However higher feed rates tend to reduce Nitrogen in larvae.
  • There is good consistency between replicates (i.e low standard error).
  • The frass has greater amounts of Boron, Calcium, Cobalt, Iron, Molybdenum, and Sodium than farmed larvae but less Manganese, Phosphorus, Nitrogen and Carbon (note: Carbon is lower for eggs only).
  • There are some metals which are not required and often detrimental to plant growth (i.e Aluminum, Lead and Cadmium).
    • Lead is not present when eggs are used as a substrate, but eggs are very high in Aluminum.
    • The pH of the product is close to neutral, and Aluminum is unavailable above pHCa4.3 but these are still very high levels.
    • Cadmium was only present in a few samples and lead appears to be well below levels considered toxic.
  • Pork substrates are higher in Boron, Cobalt, Copper, Sodium and Zinc while egg substrates are higher in Aluminum, Calcium, Magnesium and Phosphorus.
  • Carbon/Nitrogen ratio is lower in larvae (6.2) than in frass (14.4) but both are low enough to be conducive to the rapid mineralisation of Nitrogen rather than microbes having to utilise Nitrogen from the soil.

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